Photolithography, defined as a process for effecting the photographic transfer of a pattern to a surface for etching or implanting, is employed in the fabrication of myriad types of semiconductor devices, including integrated circuit (IC) devices. In general, photolithography involves the performance of a sequence of process steps, including coating a semiconductor wafer with a resist layer, exposing the coated wafer to a patterned light source, developing the resist layer, processing the semiconductor wafer through the developed resist layer, and removing the resist layer. An optical photolithography stepper apparatus, or "stepper," is typically used to expose the resist layer. An image of each layer of an IC die is formed on a small, rectangular piece of glass referred to as a reticle, or "mask." The mask is placed on the stepper and a reduced image thereof is projected onto a portion of the resist layer covering the semiconductor wafer.
Where numerous ICs are to be fabricated from a single wafer, a mask used in the fabrication of any one IC is also used in the fabrication of the other ICs from the wafer. This is accomplished by using a stepper to index, or "step," the wafer under an optical system including the mask. At each step, the photoresist is exposed by the optical system, typically with ultraviolet light, to form an aerial image of the mask in the layer of photoresist. The wafer is then removed from the stepper and the image developed. At that point, the wafer is etched to remove portions of the underlying film or implant to prepare the wafer for the next stage of material deposition or other types of etching processes. At a later stage in the fabrication process, the wafer is returned to the stepper for exposure of the wafer dies to another mask.
Clearly, it is imperative to the manufacture of high quality ICs that the performance of each stepper be maintained at an optimum level. For this purpose, most steppers currently available, such as those available from ASM Lithography, Inc., located in Eindhoven, Netherlands, are capable of generating reports comprising results of selected tests performed with the stepper, which reports are typically printed on a printer associated with the stepper. Such reports provide raw data regarding stepper performance; however, not all provide analysis of the raw stepper data. Accordingly, analysis must be performed by hand or by manually entering the data into a computer running an appropriate analysis program.
For example, one test typically performed on a stepper is a chuck flatness test for testing the flatness of a stepper's chuck. The chuck flatness test makes four scans across the wafer measuring chuck height at specified points along the scan line. Each scan measures focal deviation from one edge of the chuck to the other in one millimeter (1 mm) increments for a total of 600 data points. FIG. 1A shows the four scans 2A, 2B, 2C and 2D, respectively, and three vacuum rings 4A, 4B and 4C, respectively, Located at diameters of 150, 125 and 100 mm. A user would typically take a printout of the results of this test, which includes data for 600 data points, and manually input into a commercially available statistical modelling program, such as RS1 available from BBN Software Products Corporation of Cambridge, Mass. If the results of the test are within an acceptable range, the stepper is released to production; otherwise, the chuck is appropriately adjusted and the test is repeated. Typically, the stepper is not in production use during the testing procedure, which often takes four to five hours from the time the data is obtained to the time it is analyzed and acted upon. Clearly, therefore, such testing can result in a substantial amount of down time for the stepper.
Automating the collection and analysis of data generated by a plurality of steppers in a fab would be beneficial in that it would reduce the amount of time spent manually entering the data into analysis programs, increase the rate of data transfer to analysis and decrease the number and possibility of data entry errors. In addition, automation would provide a foundation for a paperless fab. Automation would also free up engineering time spent in rectifying data entry errors. The overwhelming amount of data collected requires the streamlining of data analysis needed for timely continuous improvement and control.
In this regard, at least one program, in particular, a Virtual Paperless Equipment Logging System hereinafter referred to as "VPELS," has been developed which collects the raw stepper data, saves the data to an ASCII file, and periodically uploads the ASCII files to a network drive, at which point it may be downloaded to and printed using a printer located outside the fab itself, thereby providing a foundation for a paperless fab. In addition, commercial programs are available for use in analyzing stepper performance; however, the raw stepper data must be manually entered into one or more of the appropriate programs before it can be processed by same, making this method undesirable for the reasons set forth above with regard to engineering time. Moreover, most such programs provide for limited types of analysis, often requiring the engineer to retype the same data severed times into several different programs to obtain the desired analysis.
Therefore, what is needed is a system for automating both data collection and analysis of stepper performance data, thereby to reduce the number of man-hours spent manually analyzing and/or entering the data for analysis while providing a greater degree of flexibility with regard to the types of analysis that may be performed.